1. Field of the Invention
The invention relates to a method of reducing power for a catalytic bead or semiconductor sensor in a gas detector by operating the sensor at a temperature lower than the desired operating temperature in the absence of a combustible gas and increasing the temperature to the desired operating temperature for accurate measurement when the sensor detects a combustible gas.
2. Description of Related Art
Gas detectors including catalytic bead (pellistor) sensors or semiconductor (MOS) sensors are widely used to detect the presence of combustible gases or vapors for safety and environmental purposes and to provide a warning of potentially explosive conditions to protect life and properties.
A catalytic bead sensor typically contains two ceramic beads coated onto platinum wire coils, a sensing bead and a compensating bead. The sensing bead is impregnated with a noble metal catalyst, which promotes combustion of the combustible gases or vapors, while the compensating bead does not contain a catalyst, but compensates for environmental effects such as humidity and ambient temperature. The sensor is typically connected to two arms of a Wheatstone bridge. When a voltage is applied across the bridge, resistive heating of the platinum wire coils and hence the beads takes place. In the presence of a combustible gas or vapor, catalytic combustion takes place on the sensing bead and generates combustion heat, causing an increase in the bead temperature and thus the sensing bead wire resistance. The Wheatstone bridge measures changes in the resistance of the sensing bead wire resistance and thus provides an output signal. Detailed descriptions on this type of gas sensor can be found, for example, in U.S. Pat. Nos. 3,200,011, 3,092,799, 4,313,907 and 4,416,911, and in Mosley, P. T. and Tofield, B. C., xe2x80x9cSolid State Gas Sensorsxe2x80x9d, Adams Hilger Press, Bristol, England (1987).
A catalytic bead sensor is typically operated at a desired temperature of about 500xc2x0 C. with a power consumption of 230-350 mW for portable gas detectors. The desired operating temperature of about 500xc2x0 C. is chosen so that all combustible gases, including methane, that require the highest bead temperature can be detected, and so that the sensor is operated under diffusion-limited conditions to provide the best accuracy and stability. The runtime of a portable gas detector operated by a battery pack is typically 10-20 hours and is largely determined by battery capacity and power usage. It would be desirable to design a portable gas detector with a small battery size and a small detector size, and therefore it would be extremely desirable to operate a catalytic bead sensor at significantly reduced power.
Several approaches have been described in the prior art to reduce the operating power of the catalytic bead sensor:
1) The first approach is to design a catalytic bead sensor that consumes low power. For example, it has been reported that a catalytic sensor fabricated by silicon micro-machining techniques has power consumption as low as 60 mW. The micro-machined catalytic sensor, however, typically possesses a very low resistance to catalyst poisons and thus a short lifetime. Furthermore, commercial micro-machined catalytic sensors are rarely available at this time. Examples of such a sensor are described in Krebs, P., and Grisel, A., Sensors and Actuators B, 13-14, 155-158 (1993); U.S. Pat. Nos. 5,813,764, 5,820,922 and 5,599,584; European Patent Application EP 0,697,593A1; and PCT application WO 00/14307.
2) The second approach is to use a catalytic bead sensor that is comprised of only a sensing bead, which is expected to reduce power to half of that for a two-bead catalytic sensor at the expense of performance. A gas detector with a single bead sensor typically has a large response to ambient temperature and humidity. For example, the Model GX-2001 of RKI Instruments, Inc. (Japan) uses such a single bead catalytic sensor.
3) The third approach is to apply pulsed power to a catalytic bead sensor. Since it typically takes 2 seconds to reach the desired operating temperature, pulse operation typically allows updating the signal output once every few seconds instead of a continuously updating signal output. For example, the Model GX-2001 of RKI Instruments, Inc. (Japan) also uses pulsed power to reduce power consumption. Examples of using pulsed power for a catalytic bead sensor are described in Japanese Provisional Utility Model Publication No. 14595, Japanese Provisional Patent Publication No. 03-233699 and U.S. Pat. Nos. 4,020,480, 6,348,872.
4) The fourth approach is to use a catalytic bead sensor in conjunction with an oxygen sensor as described in U.S. Pat. No. 6,442,994. When the oxygen sensor indicates an expected oxygen concentration in ambient atmosphere, the catalytic bead sensor is turned off, and when the oxygen sensor indicates a reduced oxygen concentration, which means some of the oxygen may have been displaced by a combustible gas, the catalytic bead sensor is turned on. However, this method depends on oxygen concentration in ambient environment and thus is only an indirect detection of the presence of a combustible gas. Furthermore, some combustible gases with very small Lower Explosive Limits (LEL) can lead to only small changes in the output signal of the oxygen sensor, which are within the variation range of the oxygen sensor in ambient atmosphere. Thus, relying on variations in the oxygen sensor output is potentially dangerous.
5) The fifth approach is to use a battery management scheme that allows efficient use of battery power. Examples of the battery power managing methods are described in U.S. Pat. No. 6,252,375 and Electronic Engineering Times, 72 (2002-01-07). The Scout MultiGas Monitor of Scott Technologies, Inc. applies this approach to extend the run time up to 50 hours.
Semiconductor sensors, which are based on metal oxide semiconductors such as tin oxide, for detecting combustible gases are well known in the prior art. They rely on adsorption of a combustible gas onto a heated oxide surface with a desired temperature in the range of typically 100-500xc2x0 C. The adsorption produces an electric conduction change in the metal oxide itself, which are related to the concentration of a combustible gas in surrounding atmosphere. A semiconductor sensor is typically composed of an electric heater, two electrodes, and a metal oxide bead surrounding the heater and electrodes. Power consumption is also an important concern when this type of sensor is used in a portable gas detector.
Therefore, it is desirable to have a method of significantly reducing power for a catalytic bead sensor or a semiconductor sensor, which can be used with commercially available combustible gas sensors and allow for continuous measurement of the atmosphere to be monitored.
It is therefore an object of the invention is to provide a method to significantly reduce power consumption of a catalytic bead sensor or a semiconductor sensor.
Another object of the invention is to provide a power reduction method that allows for direct measurement of a combustible gas.
A further object of the invention is to provide a power reduction method that can be used with commercially available combustible gas sensors.
A still further object of the invention is to provide a method to operate a catalytic bead or semiconductor sensor at reduced power while retaining sensor performance in accuracy, repeatability, response time, and poisoning resistance.
A still further object of the invention is to provide a method to operate a catalytic bead sensor or a semiconductor sensor at reduced power while increasing sensor lifetime by reducing catalyst-sintering rate.
According to the invention, power reduction is accomplished by operating a sensor which utilizes a heated surface, at a temperature which is lower than a defined, desired operating temperature for a period of time during which combustible gas, as defined by an electrical parameter output of the sensor, is below a threshold level. When the combustible gas level detected by the sensor is at least the threshold level, the operating temperature of the sensor is increased to the desired operating temperature. The lower temperature is a temperature at which the sensor operates, i.e. a temperature at which the gas is oxidized on the sensor surface, but a temperature at which accuracy and/or stability of the sensor is not adequate for normal operation.
Adjustment of the operating temperature of the sensor is accomplished by varying the parameters of the electrical supply to the sensor. Thus, voltage, current, or power is supplied, which is lower than the voltage, current, or power necessary to raise the sensor to the desired operating temperature.
The sensor of the invention may include both a sensing element and a compensating element, or may include only a sensing element.
According to an aspect of the invention, power reduction is accomplished by operating a sensor with pulses of electricity, where the duration of the pulse is adjusted, optionally in combination with the pulse voltage, current or power, to achieve an operating temperature which is lower than the desired operating temperature to save energy in the absence of a combustible gas or to achieve the desired operating temperature in the presence of a combustible gas.
According to another aspect of the invention, a sensor is calibrated at the desired operating temperature, and preferably also calibrated at the lower operating temperature, with a known concentration of a combustible gas.
According to a further aspect of the invention, the sensor is periodically switched from the lower operating temperature to the desired operating temperature to clean the sensor surface by removing adsorbed compounds.